The present invention, generally, relates to a method for producing circuit board assemblies with very fine pitch leads of surface mount components connected to pads on the circuit board and, more particularly, to pad geometry, solder volumes used in such circuit board assemblies, and stencil geometry used to supply solder paste.
Recently in the manufacture of circuit boards and cards, the making of electrical connections between chip carriers and circuit board substrates has encountered problems, and solutions have been evasive.
The increasingly fine pitch of component leads is causing a corresponding increase in the number of open circuit defects found during test of the circuit boards. Investigations indicate that the cause is from a failure of some leads to become connected, that is, some leads do not make contact with the solder on some of the pads.
The planarity of the attachment site and the relative planarity of the leads must be held to deviations less than the solder paste height or resulting solder metal height on the pads in order to reliably attach leaded surface mount components.
The clearest solution to this problem is to increase the volume of solder on the pads in order to increase the solder height during reflow, but when tried, such increase has produced other problems. It was found that the increase in the solder volume caused a type of defect called xe2x80x9csolder bridgexe2x80x9d, because of the very close spacing between the pads.
On very elongated solder pads, too much solder causes solder bulges which join together to form bridges between the pads. Thus, solder volumes have been kept sufficiently low to prevent any such bulging. Also, the length-to-width ratio of solder pads has been kept low to avoid such bridging.
The use of solder pastes at all has been questioned in the surface mount assembly of fine pitch packages also because of increased demands on post attachment rework requirements to remove solder bridges. However, an increase in the quantity of the solder paste is not the sole cause of solder bridge defects, but rather, the dimensions of the pads are involved also.
This problem does not exist only during the assembly process, but rather, it arises any time when connecting a fine pitch lead with a pad on a circuit card, whether it is during the initial assembly or during re-work.
U.S. Pat. No. 5,385,290 to Degani granted Jan. 31, 1995, describes a relationship between solder quantity and particle size and suggests an improvement is available through a use of a particular size of solder particles.
U.S. Pat. No. 5,346,118 to Degani et al. granted Sep. 13, 1994, describes a method of forming solder bumps on metal pads using a specific stencil configuration, and suggest that solder bumps thus formed will produce reliable solder joints.
U.S. Pat No. 5,180,097 to Zenshi granted Jan. 19, 1993, describes a method for mounting a component on a circuit board using strips of solder applied to a group of pads that are arranged to correspond with the leads on a component.
An IBM Technical Disclosure Bulletin, volume 37, No. 06A dated June, 1994, describes a method of depositing solder paste on circuit cards through the use of stencils with particularly defined apertures for depositing smaller particle size solder pastes.
A Technical Paper, IPC-TP-901, by Morris, dated Sep. 5-7, 1990, entitled xe2x80x9cCharacterizing Solder-Pastes For The 1990sxe2x80x9d, describes results of various tests of solder pastes and their relationships with stencils, suggesting a need for better control over the manufacturing process.
U.S. Pat. No. 4,998,342 to Bonnell et al. suggests a process for surface mounting components on a circuit board.
Accordingly, the art identified above is hereby incorporated by reference.
The problem of developing open circuits during the connection of components to a circuit board or card appears to be of increased concern today as fine pitch components evolve into ultra fine pitch components and as the spacing between the pads becomes less and less.
Clearly, what is needed is a new and completely different approach to developing a solution. Such a solution is provided by the present invention, which is described in detail hereinafter.
An important object of the present invention is to provide a new and improved method that provides optimum conditions during assembly of a multi-lead component on a circuit card so that solder paste will close gaps between contact pads on the card and the respective leads on the component.
It is a principal object of the present invention to provide a process of attaching a component to a circuit board that minimizes the creation of an open circuit.
Another object of the present invention is to provide a method of attaching a component to a circuit board that minimizes the development of short circuits.
Briefly, the method of the invention includes the steps of forming a pad on a substrate at a point where an electrical connection is needed between an electric circuit of the card and a lead of an electronic component or module. Then, a preselected solder paste is deposited through a stencil formed of a predetermined thickness with apertures formed with specific, tight tolerances in order to achieve consistent and reliable results. Then the device that is to be connected is positioned so that the lead that is to be attached is located contiguous with the pad, and an electrical connection is completed by reflowing the solder paste to form a joint of solder alloy.
Other features, objects and advantages of the present invention will become more readily apparent from the following detailed description of the presently preferred embodiment.